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Brightness judgments are a key part of the primate brain's visual analysis of the environment. There is general consensus that the perceived brightness of an image region is based not only on its actual luminance, but also on the photometric structure of its neighborhood. However, it is unclear precisely how a region's context influences its perceived brightness. Recent research has suggested that brightness estimation may be based on a sophisticated analysis of scene layout in terms of transparency, illumination and shadows. This work has called into question the role of low-level mechanisms, such as lateral inhibition, as explanations for brightness phenomena. Here we describe experiments with displays for which low-level and high-level analyses make qualitatively different predictions, and with which we can quantitatively assess the trade-offs between low-level and high-level factors. We find that brightness percepts in these displays are governed by low-level stimulus properties, even when these percepts are inconsistent with higher-level interpretations of scene layout. These results point to the important role of low-level mechanisms in determining brightness percepts.

The basal ganglia provide a major neural system through which the cortex affects behavior. Most notable among these effects are those related to the voluntary control of movement as seen in neurodegenerative disorders like Parkinson’s disease (PD). Well known tests of visual perception in PD “explicitly” measure object recognition (a high-level visual process) but “implicitly” rely on intact mid-level visual processes like grouping and figure-ground segmentation to structure the image. Hence, exploring the importance of the basal ganglia in perceptual organization (PO) abilities by examining the specific impairments incurred with the damage of such a vital structure is imperative. Therefore, this study attempted to investigate PD performance in tasks in computerized classic gestalt perception experiments with the aim of identifying any mid-level visuo-perceptual deficits. Differences were observed in the grouping by proximity dot counting task but not in other tasks that involved figure-ground segregation, part detection in embedded contexts or shape discrimination. / M.S. / Damage to the basal ganglia, a group of structures in the subcortical part of the brain (below the cerebral cortex), has long been associated primarily with Parkinson’s Disease (PD), a neurological disorder that manifests with symptoms like muscle rigidity and tremors. While several key visual and perceptual problems have also been connected to this area, very few studies have tried to describe the mechanisms by which PD functionally alters their ability to perceive the visual world. Hence, this study attempted to investigate PD performance in computerized classic perception experiments with the aim of exploring mechanisms that organize incoming visual information to structure the image called perceptual organization (PO). Differences were observed in tasks that tested their ability to group “dots” when they are varied by proximity to each other but not in other tasks that involved their ability to segregate figures from the ground, detect parts of shapes in embedded contexts or discriminate between shapes.

In an attempt to find meaningful predictors of the ability to interpret Rorschach protocols by clinicians, a paradigm change (Kuhn, 1962) was instigated by using as predictors the scores of the perceptual organizational abilities of 30 subjects, and their ratings of favorableness toward the Rorschach in terms of its usefulness as a clinical tool. The subjects were first year, graduate psychology students, and the Haptic Visual Discrimination Test (HVDT) was the instrument used to measure perceptual organization. A multiple linear regression analysis was computed, and the data supported the hypothesis that perceptual organization and favorableness are of significant predictive value (R = .54, F(2, 27) = 5.43, p = .01). The standardized beta for usefulness was .47 (p = .008) and the HVDT beta was .33, (p = .05). The results were interpreted as applying to Rorschach validity research methodology and pedagogy.

Nelson and Palmer (2007) concluded that figures/figural properties automatically attract attention, after they found that participants were faster to detect/discriminate targets appearing where a portion of a familiar object was suggested in an otherwise ambiguous display. We investigated whether these effects are truly automatic and whether they generalize to another figural property-convexity. We found that Nelson and Palmer's results do generalize to convexity, but only when participants are uncertain regarding when and where the target will appear. Dependence on uncertainty regarding target location/timing was also observed for familiarity. Thus, although we could replicate and extend Nelson and Palmer's results, our experiments showed that figures do not automatically draw attention. In addition, our research went beyond Nelson and Palmer's, in that we were able to separate figural properties from perceived figures. Because figural properties are regularities that predict where objects lie in the visual field, our results join other evidence that regularities in the environment can attract attention. More generally, our results are consistent with Bayesian theories in which priors are given more weight under conditions of uncertainty.

Little is known about how healthy aging impacts figure-ground (FG) perception, which is critical in organizing complex visual input into coherent figures. Inhibitory-suppression is thought to be critical in resolution between competing FG interpretations. Given the age-related changes to inhibitory cortical mechanisms, my thesis investigated the hypothesis that processes underlying FG organization are impaired in aging. We explored the effects of age on the convexity context effect (CCE): the tendency to perceive convex regions as figural [i.e., P(Convex=Figure)] increases with surrounding convex and concave regions. In Chapter 2, we observed that adding context by increasing region number, increased P(Convex=Figure) in younger and older observers, but this CCE was significantly reduced in older adults. Reducing competition between perceptual interpretations by heterogeneously colouring convex regions, thereby invalidating the alternative {Concave=Figure} stimulus interpretation, greatly increased P(Convex=Figure) in older observers. These results supported the notion that aging impairs FG organization, particularly in relatively high competition contexts. In Chapter 3, we explored the possibility that reduced presumption of depth in our stimuli might explain the reduced the CCE in older observers by adding texture motion to region fills consistent with depth in the stimulus. The results were inconsistent with the hypothesis, but supported the reduced inhibitory-suppression hypothesis of the age effect. In Chapter 4, we explored the neural activation associated with the processing mechanisms underlying the resolution of FG competition. The results indicated that the N250 in younger observers but not older observers is sensitive to high vs. low competition contexts. Furthermore, individual differences in this N250-competition effect were correlated with the behavioural effect of competition. Our findings are consistent with the hypothesis that the processing underlying FG organization is impaired in senescence, which may stem from reduced efficacy of the neural architecture supporting this ability. Various implications of this work are discussed. / Dissertation / Doctor of Philosophy (PhD)

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<p>Abstract</p><p>Title: Serial recall of two-voice lists: Words are not perceptually organized by voice.</p><p>The present study examined how voice affects the perceptual grouping of words in short-term memory that are auditory presented. Three different voice-lists were used: single-voice, alternating-voice and split-voice. In single-voice lists all words were presented in the same voice. In alternating-voice lists words were presented alternately in a male and a female voice. Finally, in split-voice lists words were presented grouped four and four in a male and a female voice. It was hypothesized that recall would be best for split-voice lists, especially at serial position 4 and 5. This was not confirmed. However, in line with earlier investigations, a recall advantage of single-voice lists was found in comparison with recall of alternating-voice lists at the beginning of the serial position curve. Taken together, these results suggest that words are not perceptually organized by voice in short-term memory. Like earlier studies this study found that recall of single-voice lists was better than alternating-voice lists at the first serial position, a result that yet has to be explained.</p><p>Keywords: Multi-voice lists, serial recall, short-term memory, perceptual organization</p>

Abstract Title: Serial recall of two-voice lists: Words are not perceptually organized by voice. The present study examined how voice affects the perceptual grouping of words in short-term memory that are auditory presented. Three different voice-lists were used: single-voice, alternating-voice and split-voice. In single-voice lists all words were presented in the same voice. In alternating-voice lists words were presented alternately in a male and a female voice. Finally, in split-voice lists words were presented grouped four and four in a male and a female voice. It was hypothesized that recall would be best for split-voice lists, especially at serial position 4 and 5. This was not confirmed. However, in line with earlier investigations, a recall advantage of single-voice lists was found in comparison with recall of alternating-voice lists at the beginning of the serial position curve. Taken together, these results suggest that words are not perceptually organized by voice in short-term memory. Like earlier studies this study found that recall of single-voice lists was better than alternating-voice lists at the first serial position, a result that yet has to be explained. Keywords: Multi-voice lists, serial recall, short-term memory, perceptual organization

Current developments of hardware devices have allowed the computer vision technologies to analyze complex human activities in real time. High quality computer algorithms for human activity interpretation are required by many emerging applications, such as patient behavior analysis, surveillance, gesture control video games, and other human computer interface systems. Despite great efforts that have been made in the past decades, it is still a challenging task to provide a generic gesture recognition solution that can facilitate the developments of different gesture-based applications. Human vision is able to perceive scenes continuously, recognize objects and grasp motion semantics effortlessly. Neuroscientists and psychologists have tried to understand and explain how exactly the visual system works. Some theories/hypotheses on visual perception such as the visual attention and the Gestalt Laws of perceptual organization (PO) have been established and shed some light on understanding fundamental mechanisms of human visual perception. In this dissertation, inspired by those visual attention models, we attempt to model and integrate important visual perception discoveries into a generic gesture recognition framework, which is the fundamental component of full-tier human activity understanding tasks. Our approach handles challenging tasks by: (1) organizing the complex visual information into a hierarchical structure including low-level feature, object (human body), and 4D spatiotemporal layers; 2) extracting bottom-up shape-based visual salience entities at each layer according to PO grouping laws; 3) building shape-based hierarchical salience maps in favor of high-level tasks for visual feature selection by manipulating attention conditions of the top-down knowledge about gestures and body structures; and 4) modeling gesture representations by a set of perceptual gesture salience entities (PGSEs) that provide qualitative gesture descriptions in 4D space for recognition tasks. Unlike other existing approaches, our gesture representation method encodes both extrinsic and intrinsic properties and reflects the way humans perceive the visual world so as to reduce the semantic gaps. Experimental results show our approach outperforms the others and has great potential in real-time applications. / PhD Thesis